Parasitism can be defined as a biological interaction in which one party benefits at
the expense of another (Keeton and Gould 1986). However unlike predation, the parasite
does not kill its host. In the case of avian brood parasitism, one bird lays its eggs in the
nest of a member of its own or another species and then abandons its offspring to the care
of its new foster parents. These foster parents or 'hosts' then raise the parasitic young at
the expense of their own brood. Brood parasitic birds have reproductive advantages over
those which provide parental care for their young because: 1) parental care provided by
several fosterers allows a female to produce more successful offspring than she is capable
of rearing herself in one season, and 2) parasite offspring are usually distributed among
many host nests thereby increasing the probability that at least some offspring will escape
predation (Payne 1977).
Obligate interspecific brood parasitism, where females only parasitize the nests of
other species, is a reproductive strategy adopted by approximately 1% of all bird species
and is practiced by members of five families (Anatidae, Cuculidae, Indicatoridae, Icteridae,
and Ploceidae). The degree to which interspecific brood parasites reduce host nesting
success varies with the reproductive tactics of the parasite. For example, Common
Cuckoo chicks (Cuculus canorus) eject eggs and young nest mates from the host nest with
the help of an instinctive urge to push out of the nest anything that touches the sensitive shallow depression in the parasitic nestling's back (Lack 1968). Young African Greater
Honey-guides (Indicator indicator) stab host nestlings to death with special mandibular
hooks that drop off after two weeks of age (Friedmann 1955). In contrast, black-headed
duck hatchlings seek only protection and warmth for 1-2 days post hatching and then
leave the nest with no further cost to their host (Weller 1968). Finally, Brown-headed
Cowbird nestlings intermediately affect host reproductive success by diverting parental
resources such as food away from the host's young (Payne 1977). This loss of host fitness
results in selective pressure for host defenses against parasitism such as egg ejection
(Neudorf and Sealy 1992), clutch abandonment (Burgham and Picman 1989), or increased
nest defense early in the nesting cycle (Burgham and Picman 1989, Briskie and Sealy
1989, Neudorfand Sealy 1992). To circumvent these host responses, adaptation in brood
parasites has resulted in selective pressure for egg mimicry (Rothstein 1990), egg removal,
or shorter incubation periods (Briskie and Sealy 1990, Payne 1977).
The Brown-headed Cowbird is the most abundant and widely distributed obligate
interspecific brood parasite in North America. Although the breeding behaviour of this
bird has been widely studied, most findings are contradictory. The mating system of the
Brown-headed cowbird has been described as ranging from monogamous (Laskey 1950,
Dufty 1982a, 1982b, Yokel 1986), to promiscuous (Elliot 1980). Most studies also
suggest that cowbirds parasitize multiple host species (Friedmann 1929, p 177-188, Jones
1941, McGeen & McGeen 1968, Elliot 1977, Fleischer 1985). However, a few suggest
that some individuals may be host specialists (Walkinshaw 1949, McGeen & McGeen 1968). Few of these studies have used genetic techniques to determine the actual mating
patterns and to investigate the breeding biology of males and females in a single marked
population. The main objective of this study was to use molecular genetic DNA markers
as well as behavioural observation to study the mating system and host specificity of a
Brown-headed Cowbird population at Delta Marsh, Manitoba. More specifically, my
objectives were to: 1) determine whether DNA microsatellite markers are useful for
determining parentage in Brown-headed Cowbird populations 2) document the genetic
mating system and the patterns of host use by individual females in a population of
resident cowbirds.
My findings provide the first evidence that microsatellites are useful for high
resolution parentage analyses in brood parasitic bird species where there is no A priori
information available on male or female parentage. In addition, they are the first to
directly quantity the mating system of a Brown-headed Cowbird population and to
suggest that individual females are best described as host generalists but may be showing
some preference for host nests in one habitat over another. / Thesis / Master of Science (MSc)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/22526 |
| Date | 08 1900 |
| Creators | Alderson, Gerald Wayne |
| Contributors | Biology |
| Source Sets | McMaster University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
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